Influence of interference among parallel absorbers on acoustic characteristics of an absorbing panel

This paper proposes a modified theory for synthesizing the acoustic impedance of an absorbing panel by considering the interference among parallel absorbers. The absorbing panel comprises periodically distributed absorbers with different acoustic characteristics, and the periodic distribution allows the impedance of the panel to be characterized by that of a unit cell. However, at frequencies between the resonances of the absorbers in the unit cell, the unit-cell impedance given by traditional theory deviates significantly from that given by a finite-element model. Inspecting the flow field near the surface of the unit cell reveals that the out-of-phase flow plays an important role in the interaction among the parallel absorbers and induces the deviation. A modified theory is proposed by multiplying the original resultant impedance by a factor that considers the interaction. The modified theory is verified by numerical results for several typical absorbing panels with different patterns of unit cells and different geometrical parameters of absorbers, and experimental validation is also carried out. As further evidence for the correctness and universality of the modified theoretical model, a comparison is presented with the mutual-radiation-impedance theoretical model based on a Helmholtz resonator array panel. The results of validation on different absorbing panels and the comparison with the mutual-radiation-impedance method show that the modified theoretical model is better at predicting the absorption coefficient than is the traditional theory.